A railway car conventionally includes a car body supported on the center plates of a pair of longitudinally spaced trucks. The conical-shaped wheels of the trucks engage the respective rails of a railway track. The trucks travel a generally sinuous path along the track as the respective wheels continuously seek a centered position on a respective rail. In traveling such a sinuous path, a railway truck tends to hunt, i.e., yaw or oscillate about a vertical axis of the truck. One side frame of a truck tends to move ahead of the other which, in turn, results in the flanges of the wheels striking and rubbing against the rails, first on one side, and then on the other. Such undesirable lateral oscillations may cause excessive wheel and track wear. In addition, unstable truck hunting responses can develop if the frequency of the cyclic motion approaches resonance.
Also, during travel of a railway car, a railway car body may have the tendency to rock, i.e., oscillate about a horizontal (or roll) axis of the railway car body, independent of the truck upon which the railway car body is mounted. As the trucks of a railway car negotiate their sinuous path of travel along a railway track, the car body may move laterally in concert with the cyclic lateral movement of the truck center plates. A loaded or heavy car may tolerate such lateral oscillation. However, an empty or light car body may rock from side to side which movement can become dangerous should the frequency of the rocking approach resonance.
Efforts to control truck hunting and car body rocking include the use of side bearings which are mounted to a truck bolster on opposite sides of the center plate. Conventional side bearings are configured to maintain frictional contact between a truck and a car body. As the truck yaws, an upper portion of a side bearing slides across the underside of the railway car body. The resulting friction produces an opposing torque which acts to prevent yaw motion. For example, see U.S. Pat. No. 4,712,487 to Carlson, U.S. Pat. No. 4,090,750 to Wiebe, and U.S. Pat. No. 3,762,339 to Dwyer.
One type of side bearing employs a tube form mount. Inner and outer concentric, annular members are employed. An annular elastomeric spring member is interposed between the inner and outer members. The elastomeric spring is bonded to the outer surface of the inner member and to the inner surface of the outer member such that the elastomeric spring operates in shear to resist relative axial movement between the inner and outer members. The bearing is mounted between the truck and car body such that relative displacement between the truck and the car body causes a corresponding relative axial displacement between the inner and outer members.
In order to satisfy close tolerances and achieve faster production rates, bearings as just described are preferably formed using a transfer or injection molding process. With reference to FIG. 10, a tube form mount type bearing 100 is shown. The bearing 100 as shown is mounted in a transfer or injection mold 110 by which it has been formed. The mold 110 includes an upper mold portion 102 including a transfer pot 103 which holds a pig of elastomeric material 140. A plurality of gates or sprue passages 104 extend from the bottom of the transfer pot and communicate with the cavity defined between an inner annular member 120 and an outer annular member 130. A lower mold portion 106 seals the lower end of the cavity. An intermediate mold portion 107 supports the outer member 130 in the mold 110. The elastomeric material 140 is fed into the cavity by forcing a piston 108 downwardly as indicated by arrows 108A into the transfer pot 103. The elastomeric material 140 typically follows paths as indicated by the arrows 141.
Notably, the sprue passages 104 are gated into the working section of the elastomeric member 142. One significant problem experienced with formation of a bearing as described using the prior art method described with reference to FIG. 10 is that at the openings 105 where the sprue passages 104 terminate and meet the elastomeric member 142 (commonly referred to as the sprue location sites), the elastomeric member 142 may develop undesirable performance characteristics which degrade the overall performance of the bearing 100. More particularly, the sprue location site may be a point of crack initiation when the finished and cured part is repeatedly flexed in service. When the cured cull pad material in the transfer pot 103 is removed from the elastomeric member 142, a portion of the elastomeric material 140 which has cured within the gate 104 may remain with the elastomeric member 142 as a nub or sprue. Typically, the nub or sprue must be removed. Often, when the nub or sprue is separated from the elastomeric member 142, the removed portion tears down into the working body of the elastomeric member causing deep sprues and stress concentration which may result in a reduced flex life. Also, flow eddies at the sprue location sites may cause improper knit of the elastomeric material which likewise causes a stress concentration and may reduce the member's durability.
With reference to FIG. 11, as an alternative to terminating the sprues in the working body of the elastomeric member, it has been proposed to form a bearing 100A including an elastomeric member 142A having sprue risers 106A. The upper mold portion 102A is formed with transfer pot 103A in the upper portion thereof and plurality of recesses 152A in the lower face thereof so that the sprue passage openings 105A, and thus the sprue location sites, are at the sprue risers 106A and located above the working section of the elastomeric member 142A. The stress concentrations of the sprue location sites are localized in the low stress riser 106A so that their effect on the performance of the working body of the elastomeric member 142 is reduced. While this alternative improves on the method described above, it presents significant new problems. With reference to FIG. 11A, in service, the bearing 100A is axially compressed between a contact plate 52 and a bolster (not shown). In doing so, the contact surface engages the top of the inner member 120A and also the sprue riser 106A. Chafing of the sprue riser or deflection of the sprue riser 106A into the working body of the elastomeric member 142A by the contact surface 52 may induce stress concentrations and initiate cracks in the elastomeric member. Also, the sprue riser may be unacceptably unattractive.
Accordingly, there exists a need for an elastomeric bearing having an elastomeric member formed by transfer or injection molding wherein the sprue location sites do not present stress concentration points in the working body of the elastomeric member. Further, there exists a need for a convenient and cost-effective method for forming such an elastomeric bearing. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.